Piezoelectric/electrostrictive actuators have the advantage of precise displacement control of the order of submicrons. In particular, piezoelectric/electrostrictive actuators employing a sintered piezoelectric/electrostrictive ceramic body as a piezoelectric/electrostrictive body have the advantages of in addition to precise displacement control, high electromechanical conversion efficiency, high generative power, fast response speed, great durability, and low power consumption. Making use of these advantages, the piezoelectric/electrostrictive actuators are used for equipment such as inkjet printer heads and diesel engine injectors.
The piezoelectric/electrostrictive actuators employing a sintered piezoelectric/electrostrictive ceramic body as a piezoelectric/electrostrictive body, however, may at times suffer from the problem of a reduction in the amount of displacement at high humidities, regardless of the fact that there is no such problem at typical or ordinary humidity levels. The cause of such a reduction in the amount of displacement is considered because when a piezoelectric/electrostrictive actuator is polarized or repeatedly driven, stress is concentrated on where mechanical strength is low; such as at the grain boundary or in pores of a sintered piezoelectric/electrostrictive ceramic body, thereby forming microcracks or other defects, and subsequent possible water invasion into those defects may produce a conductive path, which consequently reduces the intensity of an electric field applied to a piezoelectric/electrostrictive film.
To prevent such a reduction in the amount of displacement at high humidities, it is effective to form a coating for covering microcracks or other defects, on the surface of a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film.
For example, Japanese Patent No. 3552013 describes a technique for improving moisture resistance by forming a coating (insulator layer 13) on the surface of a laminated vibrator (piezoelectric vibrator). Japanese Patent Application Laid-open No. 2007-175989 describes another technique for improving moisture resistance by forming a coating (protective film 100) on the surface of a laminated vibrator (piezoelectric vibrator 300).
However, although moisture resistance is improved by the formation of a coating on the surface of a laminated vibrator, the conventional techniques still have the problem of a reduced amount of displacement of a piezoelectric/electrostrictive actuator because the coating will restrain the laminated vibrator. To relax this problem, Japanese Patent Application Laid-open No. 2007-175989 has proposed that part of the coating be made of a pliant material (see paragraph [0051]); however, such a measure is insufficient to produce a satisfactory effect.
Note that this is not only the problem with piezoelectric/electrostrictive actuators but also the problem common to all piezoelectric/electrostrictive elements that include a laminated vibrator made of laminations of a piezoelectric/electrostrictive film and an electrode film.